The present invention will be described with particular reference to a film for use in packaging food. However, it will be appreciated that the film of the present invention may have applications and uses other than in the food industry and no limitation is intended thereby.
There are a number of applications for articles having a material with a liquid absorbed onto a surface thereof. Such applications include “patches” as used for slow release topical administration of therapeutic agents and food casings for release of a flavouring and/or colouring agent into the food encased therein. However, the types of materials which can be used to form such articles are strictly limited to those having suitable absorption or permeability properties for the desired liquid.
In practice, this has severely restricted the types of materials which may be used. Generally for hydrophilic liquids, the materials used are cellulosic materials and derivatives or modifications of water soluble polymers such as polyacrylamide, polyvinyl alcohol, polyvinylpyrolidone and the like. These materials are typically modified such that the articles formed therefrom are no longer water soluble but are able to absorb at least some water. The polymers may be crosslinked and/or blended or copolymerised with less hydrophilic and/or hydrophobic polymers or monomers. A particular application of a film having a liquid absorbed therein are cellulosic food casings having a smoke flavouring agent known as “liquid smoke” absorbed therein. Such casings have been developed to be able to apply a smoke flavour and/or colour to a food product as an alternative to the traditional smoking process.
The traditional smoking process involves stuffing a food to be smoked, such as a sausage, into a smoke permeable casing and hanging the stuffed food product in a smoke house in which wood was burned at low temperature to generate smoke. Smoking by such traditional techniques is labour intensive, time consuming (with smoking times being up to a number of days) and the level of smoke generated often conflicts with pollution laws.
In order to provide the benefits of a smoked product but without the drawbacks of conventional smoking techniques, liquid smoke was developed some 65 years ago. Liquid smoke is the aqueous condensate of natural wood smoke, which contains aldehydes that react with proteins in the meat resulting in browning of the meat surface. This browning can give the appearance of a naturally smoked product
Liquid smoke was originally sprayed onto meat products encased in porous natural gut or cellulose casings prior to cooking by placing in an oven. The porosity of the casings allowed the liquid smoke to penetrate the casing and into the meat. In practice, however, it has been found that with this spraying process it is difficult to provide a uniform coating to the product. Further, the liquid smoke spray tended to run down the sides of the stuffed casing and gather at the lower end thereof. This uneven coating of liquid smoke resulted in a product having an undesirable blotchy appearance.
In the light of these difficulties, much work has been conducted with a view to providing a substantially uniform distribution of liquid smoke onto the surface. Some attempts have been made to incorporate liquid smoke into the meat product itself. However, this provides either an insufficient amount of agent on the surface to provide the desired colour, or imparts an excessively smoky flavour to the product.
A more successful approach has been to incorporate liquid smoke into a cellulose casing or absorb liquid smoke onto the surface of a cellulose casing. Techniques for absorbing liquid smoke onto a cellulose casing include dipping or spraying the cellulose casing with a liquid smoke solution so that the liquid smoke permeates to the interior surface for eventual transfer to an encased product. Such external treatment can present difficulties with further processing of the cellulose casings and in particular, may interfere with shirring. Despite these difficulties, there are a number of liquid smoke treated cellulose casings on the market.
The permeability and porosity of cellulose casings which on the one hand, makes them compatible for use with liquid smoke, on the other hand introduces serious and widely recognized disadvantages. First, cellulose products are highly permeable to water vapour. This allows moisture loss during cooking and a corresponding decrease in overall weight of the product. This results in a reduced yield of product, which is disadvantageous for commercial reasons. Further, cellulose casings are also oxygen permeable which leads to spoiling and/or discolouration of the food product. As a result of this latter difficulty, food products cooked in cellulose casings must either by consumed shortly after cooking, or be removed from the casings and wrapped a second time as quickly as possible after production with an oxygen impermeable barrier casing. This rewrapping step provides an opportunity for contamination or infection of the food product that represents a loss in quality and shortening of shelf life. Further, the additional steps add to costs.
To address the disadvantages of permeable cellulose casings, water and oxygen impermeable thermoplastic casings were developed. When using casing of this type, there is negligible loss in weight during the production process or during storage and shipping. Further, the product can remain sterile provided the casings remain intact. However, as a result of the impermability of the casing, smoke in either gaseous or liquid form cannot penetrate the casing. Further, impregnation of thermoplastic casings with liquid smoke or other colouring additives has not been successful since the plastics used cannot adequately absorb and store impregnating agents. In order to impart a smoke colour to plastic encased products, it is necessary to remove the casings and apply smoke by techniques such as spraying or dipping liquid smoke or atomizing and the product repackaged. These additional steps not only increase cost but increase the risk of contamination.
Another approach has been to provide a cellulose/plastic laminate, the inner cellulose layer having a coloring agent absorbed therein and the plastic intended to provide an oxygen barrier layer. In practice however, when meats are cooked in packages formed from this material, juices collect between the meat and package. Such a condition is known as “cook-out” or purge. This is undesirable from a consumer acceptance point of view. Further, it has been observed that there is a tendency for flaking or chipping of the cooked meat surface during slicing.
There is a recognized need in the industry to provide a gas and moisture impermeable casing having a food additive such as liquid smoke absorbed therein. However, to date all attempts to solve this problem and provide a commercially acceptable product have failed for one reason or another. One approach has been to blend a liquid smoke with a resin used for the inner layer of a plastic casing. This has been unsuccessful for a number of reasons including reaction and volatilization of the liquid smoke at extrusion temperatures and delamination induced by the presence of the additive.
Another approach has been to incorporate an absorbent additive into an impermeable plastic casing material. However, such a product has yet to be adopted commercially and in tests conducted by the present inventor on one such material, have shown that there is still insufficient absorption of liquid into the film and excess liquid remains on the surface of the film.
If this excess liquid is not removed, the colour and flavour additives do not remain evenly dispersed on the film surface during subsequent processing such as shirring and stuffing. The result is non uniform transfer of colour and flavour to the surface of the product.
If the excess liquid is removed by wiping, very little of the colour and flavour additives remain absorbed in the film structure and little or no colour and flavour is transferred to the product surface.
If the casing is in the form of a flat sheet the excess liquid may be removed by applying heat to dry off the moisture leaving the colour and flavour additives as a coating on the film surface. However, heating may be difficult or not possible if the film is a heat shrink film. The casing can then be formed into a tube to encase the food product. A delicate balance is required to have the coating adhere sufficiently to the casing surface to withstand shirring and stuffing operations and yet release to the surface of the food product during processing.
If the casing is in the form of a tube, the liquid containing colour and flavour solution can be added to the inside of the tube by well known techniques known as slugging but there is no practical method to remove the excess liquid. Therefore sufficient flavour and colour cannot be added without non-uniform transfer to the product surface.
It will be appreciated that in other applications unrelated to the food industry, it may also be desirable to provide an alternative to those materials currently used in applications where it is desirable to have a liquid at least partially absorbed therein. It is therefore an object of the present invention to provide a film having a liquid at least partially absorbed therein and a method for producing such a film that may at least partially overcome the above disadvantages, or provide the public with a useful or commercial choice.